Stabilization of polyacetals



United States Patent 3,204,014 STABILIZATION OF POLYACETALS Richard Green, Livingston, NJ., assignor, by mesne assignments, to Tenneco Chemicals, Inc, a corporation of Delaware N0 Drawing. Filed Sept. 20, 1962, Ser. No. 225,114 Claims. (Cl. 260-895) tensile strength, that make them useful in the manufacdation or attack by acids. A number of materials, such as secondary and tertiary monomeric aromatic amines, superpolyamides, urea and thiourea, and hydroxyanthraquinones, have been suggested for use as stabilizers for high molecular weight polymers of formaldehyde, but none of these has proven to be entirely satisfactory for this purpose. Many of the known stabilizers do not provide the required stabilizing effect, while others impart color to the stabilized compositions.

In accordance with this invention it has been found that high molecular weight polymers of formaldehyde can be stablized against thermal degradation by incorporating in the polymer compositions a small amount of a stabilizer system comprising copolymer of vinylpyrrolidone, a phenolic antioxidant, and a dialkyl ester of 3,3-thiodipropionic acid. The resulting polyacetal compositions are characterized by excellent thermal stability and by good color and color retention.

The copolymers of vinylpyrrolidone which may be used in the stabilizer system are copolymers resulting from the polymerization of a mixture of N-vinylpyrrolidone and any of a wide variety of monoolefinic monomers copolymerizable therewith. The useful comonomers include, for example, vinylchl-oride, vinylidene chloride, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl ethers, styrene, a-methylstyrene, acrylonitrile, methacrylonitri-le, acrylic acid, methacrylic acid, maleic anhydride, alkyl acrylates, alkyl methacrylates, alkyl maleates, alkyl fumarates, alkyl crotonates, and the like. The comonomers are used in such proportions that the copolymers contain approximately to 90% of N-vinylpyrrolidone units and 10% to 70% of units derived from the other monomer. The vinylpyrrolidone copolymers may be prepared by mass, solution, or emulsion polymerization procedures in the various manners well-known to the art.

The second component of the stabilizer system is a phenolic antioxidant having a structure represented by the formula H OH wherein R represents a hydrogen atom or an alkyl group having from 1 to 3 carbon atoms and each R represents an alkyl group having from 1 to 4 carbon atoms. Illustrative of these compounds are the following: 2,2-meth- 3,204,014 Patented Aug. 31, 1965 ylene bis (4-methyl 6-tert. butylphenol), 2,2'-methylene bis (4-ethyl-6-tert. butylphenol), 4,4'-butylidene bis (3- methy1-6-tert. butylphenol), 2,2'-butylidene bis (4,6- dimethylphenol), 2,2'-butylidene bis (4-tert. butyl-6- methylphenol), 2,2'-ethylidene bis (4-methyl-6-tert. butyl-phenol), 4,4"-methylene bis (2,6-di-tert. butylphenol), 6,6-butylidene bis (2,4-xylenol), 4,4'-butylidene bis (2,5- xylenol), and the like. A single phenolic antioxidant or a mixture of two or more of these compounds may be present in the stabilizer system.

The dialkyl esters of 3,3'-thiodipropionic acid which may be used in combination with the vinylpyrrolidone copolymer and phenolic antioxidant have the formula wherein each R" represents an alkyl group having a structure represented by from 8 to 20 carbon atoms, a cycloalkyl group, or a hydroxyalkyl group having from 1 to 4 carbon atoms. These esters include, for example, dioctyl 3,3'-thiodipropionate, didecyl 3,3-thiodipropionate, diacetyl 3,3'-thiodipropionate, distearyl 3,3'-thiodipropionate, dieicosyl 3,3'-thiopropionate, lauryl stearyl 3,3-thiodipropionate, dicyclohexyl 3,3-thiodipropionate, di (4- ethylcyclohexyl) 3,3'-thiodipropionate, di (hydroxymethyl) 3,3-thiodipropionate, di (hydroxyethyl) 3,3- thiodipropionate and di (hydroxybutyl) 3,3'-thiodipropionate. The preferred esters are those in which R" represents an alkyl group having from 12 to 18 carbon atoms, such as dilauryl 3,3'-thiodipropionate, dicetyl 3,3- thiodipropionate, and distearyl 3,3'-thiodipropionate.

The stabilized polyacetal compositions generally contain approximately 0.1% to 10% by weight of the stabilizer system based on the weight of the high molecular Weight polymer of formaladehyde, with amounts of ap proximately 1% of 5% being preferred. The Weight ratios of the three components comprising the stabilizer system can also be varied. The stabilizer system generally contains approximately 5 to 30 parts of the vinylpyrrolidone copolymer and 1 to 20 parts of the phenolic antioxidant per part of the thiodipropionate ester. The preferred ratio of stabilizer components is approximately 10 to 20 parts of the vinyl .pyrrolidone copolymer and 2 to 5 parts of the phenolic antioxidant per part of the thiodipropionate ester. It is particluarly preferred to use approximately 15 parts of copolymer and approximately 3 parts of the phenolic antioxidant per part of the thiodipropionate ester.

The combination of vi-nylpyrnolidone copolymer, phenolic antioxidant, and dialkyl ester of 3,3-thiodipropionic acid as herein described can be used to stabilize a wide variety of high molecular weight polymers of formaldehyde compositions against thermal degradation. Any of the normally solid, high molecular weight formaldehyde homopolymers and copolymers can 'be stabilized in accordance with this invention. As used herein, the term high molecular Weight polymer of formaldehyde includes both linear and branched-chain homopolymers and copolymers of formaldehyde having a molecular weight in the range from about 15,000 to about 200,000 and a melting point in excess of C., the predominant structural feature of both homopolymers and copolymers being the recurring oxymethylene units (OCH in the polymer chain. Among the most important homopolymers are a,w-polyoxymethylene glycol, a,w-polyoxymethylene dicarboxylates (such as the diacetate or dipropionate), and a,w-polyoxymethylene diethers (such as the methyl, ethyl, or B-hydroxyethyl ethers), all of which may have w-substituted polyoxymethylene branched chains at one or more points in the main polymer chain. The formaldehyde copolymers include both linear and branched-chain polymers in which the recurring oxymethylene units (OCH in the polymer chains are periodically or randomly interrupted by other oxyalkylene units containing two or more vicinal carbon atoms, or by other units introduced by copolymerizing other monomers, such as u-butyrolactone, phthalide or isocyanic acid, with substantially anhydrous formaldehyde or with it trimer, trioxane.

The stabilizer system may be incorporated into the high molecular weight polymers of formaldehyde in any convenient manner. One method for the preparation of 'the polyacetal compositions of this invention involves Table I Stabilizer Components Percent Ex. Percent 4,4- Percent weight Color No. butylidene dilauryl loss copolymer bis (3-mcthylthiodi- 6-tert.butylpropionate phenol) 1A 1.5% N-vinylpyrrolidone-vinyl acetate copolymer 0.3 0.1 0.95 Light (:70). yellow 1B EIiN-v1nylpyrrolidone-vmyl acetate copolymer 0.3 0.1 0. 04 D0. 1C 1.536 1gl vinylpyrrolidone-acrylonitrile copolymer 0.3 0.1 0.5 Yellow 11).--. 1.5% N-vinylpyrrolidone-acrylamide copolymer 0.3 0.1 0.4 Light (90:10 yellow 1E 271830llLvinylpyrrolidone-vmyl acetate copolymer 0 0 6.2 Yellow 1F 29% 0N-v inylpyrroliclone-viuyl acetate copolymer 0 0 11.7 Brouiln- 7 ye 0w 1G $7690 N-vinylpyrrolidone-acrylonitrile copolymer 0 0 8.9 Deepll ye ow 1H 2Z(,90 15-vinylpyrrolidone'acrylamide copolymer 0 0 7.3 Yellow 1I None i o 0 22.6

dissolving the stabilizer components in a volatile solvent,

adding to the iigh molecular weight polymer of formaldehyde an amount of the solution that will provide the desired amount of the stabilizer in the composition, and removing the solvent by evaporation. Alternatively, the stabilized polyacetal compositions of the invention may be prepared by milling the dry solids together or by disssolving both the high molecular weight polymer of formaldehyde and the stabilizer system in a common solvent and EXAMPLE 1 A series of stabilized polyacetal compositions was prepared by the following procedure: To one part of polyoxymethylene diacetate (molecular weight-30,000) were added 1.5 parts of a 1% solution of a vinylpyrrolidone copolymer in acetone, 0.3 part of a 1% solution of 4,4'-butylidene bis (3-methyl-6-tert. butylphenol) in acetone, and 0.1 part of a 1% solution of dilauryl thiodipropionate in acetone. The resulting mixtures were air dried to remove From the data in Table I it will be seen that the compositions which contained the novel ternary stabilizer system were far more stable when heated in an oxidizing atmosphere than either the unstabilized compositions or the compositions which contained 2% of the vinylpyrrolidone copolymer as stabilizer. In addition, those compositions which contained the stabilizer system were lighter in color than those which contained approximately the same pro portion of the vinylpyrrolidone copolymer.

EXAMPLE 2 A series of stabilized polyacetal compositions was prepared by adding to 1 part of polyoxymethlene diacetate (molecular weight50,000) 1.5 parts of a 1% solution of a vinylpyrrolidone copolymer in acetone, 0.3 part of a 1% solution of 4,4-butylidene bis (3-methyl-6-tert. butylphenol) in acetone, and 0.1'part of a 1% solution of dilauryl thiodipropionate in acetone. Following re- .moval of the acetone by evaporation, the resultant compositions were heated in an oxidizing atmosphere at 222 C. for 30 minutes, cooled to room temperature, and weighed. The stabilizer systems which were used and the properties of the stabilized polyacetal compositions are given in Table II. For comparative purposes, this table also includes data on an unstabilized composition and on compositions which contained as stabilizer only the vinylpyrrolidone copolymer.

Table II Stabilizer Components Percent Ex. Percent 4,4- Percent weight Color N o. butylidene dilauryl loss copolymer bis (S-methylthiodi- 6-tert.bntylpropionate phenol) 2A 1.5% N-vinylpyrrolidone-ethyl acrylate copolymer. 0.3 0.1 3.0 Light ellow 2B 1.5% N-vinylpyrrolidone-styrene copolymer 0. 3 0. 1 11. 1 Light brown 20 2% N-vinylpyrrolidone-ethyl acrylate copo lymer 0 0 36. 0 Light ellow 2D 2% N-vinylpyrrolidone-styrene copolymer 0 0 33. 8 Brt iwn 2E N 0 0 35.6

The polyacetal compositions of this invention may, if desired, contain other stabilizers, such as ultraviolet light absorbers, as well as plasticizers, fillers, pigments, solvents, dyes, etc., in the amounts ordinarily employed for the purposes indicated.

The stabilized polyacetal compositions may be converted to films, fibers, molded articles, and the like by melt-extrusion, injection-molding, compression-molding, and other fabrication methods known in the art.

What is claimed is:

1. A thermally stable polyacetal composition comprising a high molecular weight polymer of formaldehyde and containing from about 0.1% to about by weight of a stabilizer system comprising (a) a copolymer resulting from the polymerization of a mixture of 30% to 90% by weight of N-vinyipyrrolidone and 10% to 70% by weight of a monoolefinic monomer copoiymerizable therewith; (b) a phenolic antioxidant having the fomula HO OH wherein R represents'a member selected from the group consisting of a hydrogen atom and alkyl groups having from i to 3 carbon atoms and each R represents an alkyl group having from i to 4 carbon atoms; and (c) a diester of 3.3'-thiodipropionic acid having the formula wherein R" represents a member selected from the group consisting of alkyl groups having 8 to 20 carbon atoms, cyclo-alkyl groups, and hydroxyalltyl groups having to 1 to 4 carbon atoms in the amounts of approximately 5 to 30 parts by weight of said polyvinylpyrrolidone copolymer and l to 20 parts by weight of'said phenolic antioxidant per part by weight of said thiodipropionic acid ester, all percentages except those designating the composition of the vinylpyrrolidone copolymer being based on the weight of the formaldehyde polymer in the polyacetal composition.

2. A thermally stable polyacetal composition comprising a high molecular weight polymer of formaldehyde and containing from about 1% to about 5% by weight of a stabilizer system comprising (a) a copolymer resulting from the polymerization of a mixture of 30% to 90% by weight of N-vinyipyrrolidone and 10% to 70% by weight of a monooiefinic monomer copoiymerizable therewith; (b) a phenolic antioxidant having the formula 110 H OK i wherein R represents a member selected from the group consisting of a hydrogen atom and allryl groups having from l to 3 carbon atoms and each R represents an alkyl group having from 1 to 4 carbon atoms; and (c) a diester of 3,3'-thiodipropionic acid having the formula HO OH wherein R represents a member selected from the group consisting of a hydrogen atom and alkyl groups having from 1 to 3 carbon atoms and each R represents an alkyl group having from 1 to 4 carbon atoms; and (c) a diester of 3,3'-thiodipropionic acid having the formula B(cH|CHs QR")| wherein R" represents a member selected from the group consisting of alkyl groups having 8 to 20 carbon atoms, cycloalkyl groups, and hydroxyalkyl groups having 1 to 4 carbon atoms in the amounts of approximately 10 to 20 parts by weight of said poiyvinylpyrroiidone copolymer and 2 to 5 parts by weight of said phenolic antioxidant per part by weight of said thiodipropionic acid ester, all percentages except those designating the composition of the vinylpyrrolidone copolymer being based on the weight of the polyoxymethylene' diacetate in the polyacetal composition.

4. The thermally stable polyacetal composition of claim 3 wherein the copolymer is a copolymer of N-vinylpyrrolidone and vinyl acetate.

5. The thermally-stable polyacetal composition of claim 3 wherein the copolymer is a copolymer of N-vinylpyrrolidone and acryionitriie.

6. The thermally-stable polyacetal composition of claim 3 wherein the copolymer is a copolymer of N-vinylpyrrolidone and acrylamide.

7. The thermally-stable polyacetal composition of claim 3 wherein the copolymer is a gopoiymer of N-vinyipyrrolidone and styrene.

8. The thermally-stable polyacetal composition of claim 3 wherein the copolymer is a copolymer of N-vinylpyrrolidone and ethyl acrylate.

9. The thermally-stable polyacetal composition of claim 3 wherein the phenolic antioxidant is 4,4-butylidene bis (3-methyl-6-tert. butyiphenoi).

10. The thermally-stable polyacetal composition of claim 3 wherein the diester of 3,3'-thiodipropionic acid is diiauryi 3,3'-thiodipropionate.

11. The thermally-stable polyacetal composition of claim 3 wherein the diester of 3,3-thiodipropionic acid is distearyl 3,3'-thiodipropionate.

12. A thermally-stable polyacetal composition comprising polyoxymethylene diacetate having a number average molecular weight of at least 20,000, as determined viscosimetricaily, and containing a stabilizer system comprising (a) approximately 1.5% of a copolymer resulting from the polymerization of a mixture of 30% to by weight of N-vinyipyrroiidone and 10% to 70% by weight of a monoolefinic monomer copoiymerizable therewith; (b) approximately 0.3% of 4,4'-butylidene bis (3-methyl-6-tert. butylphenoi); and (c) approximately 0.1% of dilauryl thiodipropionate, all percentages except those designating the composition of the vinylpyrrolidone copolymer being based on the weight of the polyoxymethylene diacetate in the polyacetal composition.

13. The thermally-stable resinous composition of claim 12 wherein the copoiymer is a copolymer of N-vinyh pyrrolidone and vinyl acetate.

14. The thermally-stabie resinous composition of claim 12 wherein the copoiymer is a copolymer of N-vinylpyrroiidone and acryionitriie.

15. The thermally-stable resinous composition of claim 12 wherein the copoiymer is a copolymer of N-vinylpyrrolidone and acrylamide.

8 References Cited by the Examiner UNITED STATES PATENTS 2,966,476 12/60 Kralovec et al. 26045.9 3,033,814 5/62 Thoiatrup 260-4595 3,103,499 9/63 Doice et a1. 260-45.!)

MURRAY TILLMAN, Primary Examiner.

WILLIAM H. SHORT, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,204,014 August 31, 1965 Richard Green It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, lines 5 and 6, for "butylphenol" read butylphenol lines 21 and 22, for "diacetyl" read dicetyl line 23, for "3,3-thiopropionate" read 3,3J-thiodipropionate line 36, for "of" read to line 43, for "vinyl p rrolidone" read vinylpyrrolidone line 46, after "of" i sert the line 49, after "of" insert a line 53, strike out "compositions"; column 3, line 35, for "iigh" read high column 6, line 11, after "of" insert a line 31, for "polyvinylpyrrolidone" read N-vinylpyrrolidone line 39, for "thermally stable" read thermally-stable Signed and sealed this 17th day of May 1966.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. A THERMALLY STABLE PLYACETAL COMPOSITION COMPRISING A HIGH MOLECULAR WEIGHT POLYMER OF FORMALDEHYDE AND CONTAINING FROM ABOUT 0.1% TO ABOUT 10% BY WEIGHT OF A STABILIZER SYSTEM COMPRISING (A) A COPOLYMER RESULTING FROM THE POLYMERIZATION OF A MIXTURE OF 30% TO 90% BY WEIGHT OF N-VINYLPYRROLIDONE AND 10% TO 70% BY WEIGHT OF A MONOOLEFINIC MONOMER COPOLYMERIZABLE THEREWITH; (B) A PHENOLIC ANTIOXIDANT HAVING THE FORMULA 